Use of light metals such as aluminum (Al) alloy and high-strength steel sheets for automobile members has been suggested for the purpose of reducing weight in order to improve automobile fuel consumption. The light metals such as Al alloy offer the advantage of high specific strength; however, since they are much more expensive than steel, their applications are limited to special applications. Thus, there may be a need to increase the strength of steel sheet to promote cost decreases and automobile weight reductions over a wider range.
Since increasing the strength of a material typically causes deterioration of moldability (processability) and other material characteristics, the key to developing high-strength steel sheet is the extent to which strength can be increased without deteriorating material characteristics. Since characteristics such as stretch flangability, ductility, fatigue durability and corrosion resistance are important characteristics that are preferred for a steel sheet used for inner plate members, structural members and underbody members, and how effectively these characteristics can be balanced with high strength on a high order may be important.
For example, Japanese Unexamined Patent Applications, First Publication Nos. 2000-169935 and 2000-169936 describes a transformation-induced plasticity (TRIP) steel in which moldability (ductility and deep drawability) are dramatically improved as a result causing the occurrence of TRIP phenomenon during molding by containing residual austenite in the microstructure of the steel in order to achieve both high strength and various advantageous characteristics, especially moldability.
The steel sheet obtained in such manner can demonstrate a breaking elongation in excess of 35% and superior deep drawability (limiting drawing ratio—LDR) due to the occurrence of TRIP phenomenon by the residual austenite at a strength level of about 590 MPa. However, amounts of elements such as C, Si and Mn should inevitably be reduced in order to obtain steel sheet having strength within the range of 370 to 540 MPa, and when the amounts of elements such as C, Si and Mn are reduced to realize the strength within the range of 370 to 540 MPa, there is the problem of likely being unable to maintain amount of residual austenite required for obtaining TRIP phenomenon in the microstructure at room temperature. In addition, the emphasis of the above art is not placed on improving stretch flangability. Thus, it may be difficult to apply high-strength steel sheet having strength of 540 MPa or higher to a member in which steel sheet having strength on the order of 270 to 340 MPa is currently used, without first improving operations and equipment used during pressing. The likely solution has been to use steel sheet having strength of about 370 to 490 MPa. On the other hand, a preference for a reduction of gauges has been increasing in order to achieve reduction in weight for an automobile body, and it is therefore it may be important to reduce the weight for the automobile body to maintain pressed product strength as much as possible, based on the premise of reducing gauges.
Bake-hardening (BH) steel sheet has described as a solution to these problems because it generally has a low strength during press molding and improves the strength of pressed products as a result of introducing stress due to pressing and subsequent baking finish treatment.
It may be effective to increase solute C and solute N so as to improve bake hardenability. However, increases in such solute elements present in the solid solution can worsen aging deterioration at normal temperatures. Consequently, it may be important to develop a technology that can allow both bake hardenability and resistance to aging deterioration at normal temperatures.
On the basis of the preferences described above, Japanese Unexamined Patent Applications, First Publication Nos. H10-183301 and 2000-297350 describe technologies for realizing both bake hardenability and resistance to aging deterioration at normal temperatures, in which bake hardenability is improved by increasing the amount of solute N, and the diffusion of solute C and solute N at normal temperatures is inhibited by an effect of increasing grain boundary surface area caused by grain refining of crystal grains.
However, the grain refining of crystal grains generally has the risk of deteriorating press moldability, while the addition of solute N has the risk of causing aging deterioration. In addition, despite the need for superior stretch flangability in the case of applying to underbody members and inner plate parts, since the microstructure includes ferrite-pearlite having a average crystal grain size of 8 μm or less, it is unsuitable with respect to stretch flangability.